Modern gas or combustion turbines must satisfy the highest demands with respect to reliability, weight, power, economy, and operating service life. In the development of such turbines, the material selection, the search for new suitable materials, as well as the search for new production methods, among other things, play a role in meeting standards and satisfying the demand.
The materials used for gas turbines may include titanium alloys, nickel alloys (also called super alloys) and high strength steels. For aircraft engines, titanium alloys are generally used for compressor parts, nickel alloys are suitable for the hot parts of the aircraft engine, and the high strength steels are used, for example, for compressor housings and turbine housings. The highly loaded or stressed gas turbine components, such as components for a compressor for example, are typically forged parts. Components for a turbine, on the other hand, are typically embodied as investment cast parts.
Although investment casting is not a new process, the investment casting market continues to grow as the demand for more intricate and complicated parts increases. Because of the great demand for high quality, precision castings, there continuously remains a need to develop new ways to make investment castings more quickly, efficiently, cheaply and of higher quality.
Conventional investment mold compounds that consist of fused silica, cristobalite, gypsum, or the like, that are used in casting jewelry and dental prostheses industries are generally not suitable for casting reactive alloys, such as titanium alloys. One reason is because there is a reaction between mold titanium and the investment mold.
There is a need for a simple investment mold that does not react significantly with titanium and titanium aluminide alloys. Approaches have been adopted previously with ceramic shell molds for titanium alloy castings. In the prior examples, in order to reduce the limitations of the conventional investment mold compounds, several additional mold materials have been developed. For example, an investment compound was developed of an oxidation-expansion type in which magnesium oxide or zirconia was used as a main component and metallic zirconium was added to the main constituent to compensate for the shrinkage due to solidification of the cast metal. There is thus also a need for simple and reliable investment casting methods which allow easy extraction of near-net-shape metal or metal alloys from an investment mold that does not react significantly with the metal or metal alloy.
Certain references describe using calcium hexaluminate and calcium aluminate cement have been disclosed. For example, references such as U.S. Pat. Nos. 3,269,848 and 3,312,558 to Miller disclose the production of calcium hexaluminate and the production of shapes of calcium hexaluminate and calcium aluminate cement, including slip casting molds. However, such references do not disclose the use of calcium hexaluminate as a component of a casting mold for reactive alloy articles and certain complex articles such as turbine components.
Other references, such as European Patent Application No. 1178023 A1 to Gnauck et al., disclose a high density refractory material containing calcium hexaluminate produced by combining a mixture of aluminum oxide with calcium oxide and a sintering aid. The calcium hexaluminate is produced to have a bulk specific density greater than 90 percent. However, these references do not disclose the use of calcium hexaluminate as a component of a casting mold for reactive alloy articles and turbine components.
Other references, such as U.S. Patent Application No. US 2008/0175990 to McGowan et al., disclose the use of calcium hexaluminate with calcium aluminate cement. Such references describe methods that involve the use of calcium hexaluminate for improving the insulating character and/or penetration resistance of a liner in contact with at least one of an alkali environment and/or alkaline environment. This method comprises lining a surface that is subject to wear by an alkali environment and/or an alkaline environment with a refractory composition comprising a refractory aggregate consisting essentially of a calcium hexaluminate clinker, and wherein the hexaluminate clinker has from zero to less than about fifty weight percent mayenite. Such references also describes methods for making articles of stable calcium hexaluminate by starting with alpha alumina and calcium oxide. The articles so produced can also possess burnout material such that the shapes produced have porosity between 50% and 70%. The examples disclosed in the references involve the use of calcium aluminate cement in conjunction with calcium hexaluminate, but at very low concentrations of calcium aluminate cement. For example, the references describe the weight concentration of calcium aluminate cement to calcium hexaluminate to range from 1:4 to 1:14.